Flat surface not being shaded consistently across quads
While deactivating the "Auto Smooth" helps at the moment, i believe that something else happened and messed the normals up. 

In my humble opinion i think that the surface should be fine even if the Normals are set to "Auto Smooth", in that regard i checked and found that if you press Alt+N and use the "Set From Faces" option, the surface normals are corrected.

Here the visual confirmation: